Yellow bristle grass, the persistent summer weed that wrecks productive pasture, has spread to South Taranaki.

The weed first appeared in Tikorangi and Lepperton about three years ago and has since spread to other areas of North Taranaki, including Urenui, Onaero and Okato, and more recently on the upper reaches of Egmont Rd.

A lot of yellow bristle grass was now being found in South Taranaki, and it was also being found at altitude, DairyNZ Taranaki regional leader Katrina Knowles said.

"The biggest risk now is with maize harvesting and yellow bristle grass seed blowing off trucks transporting maize.

"Farmers should feel comfortable about refusing to take the outer round of the maize crop if it's infested with yellow bristle grass."

She hoped AgResearch scientist and yellow bristle grass specialist Trevor James would be able to talk to the region's district councils about managing the weed on roadsides to prevent it spreading to pasture.

Taranaki Federated Farmers sharemilkers' employers chairman Othmar Hebler said many South Taranaki farmers didn't realise that yellow bristle grass was on their doorstep. There was a lot around the Ohangai and Manutahi areas.

Taranaki Federated Farmers president Bronwyn Muir said the organisation was concerned about the spread of the weed, and was working with the Taranaki Regional Council to include it in its pest management plan. It was also making submissions to the South Taranaki District Council.

District councillor Ian Wards has taken council officers on a tour to show them the extent of the weed on road verges around Ohangai and Manutahi.

Wards said farmers needed to be on alert for the weed the roadsides, and spray it to prevent it infesting pasture.

The persistence of the weed, which crowds out ryegrass and clover, made it difficult to manage, but the herbicide Puma S was effective before the seed head emerged, AgResearch scientist Katherine Tozer said.

It remained viable in silage stacks for up to three months, and survived in the rumen of cattle and in effluent ponds.

AgResearch and DairyNZ have produced a guide to help farmers identify yellow bristle grass and show them ways to get rid of it.